This invention relates to a method and device for measuring depth of field that uses a charged couple device (CCD) and a lens to scan a selected and slanted test chart to obtain focusing characteristics thereby to derive depth of field distribution range for getting optimum focus point of the CCD and lens.
Scanner is a widely used computer peripheral device nowadays for input image data into the computer. The explosive growth and popularity of Internet in recent years has further expanded the usage of scanner. It becomes an essential device for producing image and text Web pages for Internet applications. Scanner function, quality and versatility become very important. Focus adjustment capability and resolution of the scanner are among the key features that have been highly valued and demanded by users.
During scanner production process, scanner focus adjustment is a critical operation which will greatly affect scanner quality. FIG. 1 shows a conventional technique of testing and tuning scanner focus. It uses a pair of holder 11 to support a test chart 10 above the transparent scanning object board 12. Below the scanning board 12, there is a scanning chassis 20 which includes at least one reflection mirror 21, a lens 22 and a CCD 23. During testing process. The scanning chassis 20 is moved to scan the test chart to get a test image. Then another height of holder 11 is used to support the test chart 10 for obtaining another testing image. After a selected number of testing have been performed with different holder heights, a distribution line graph of Modulation Transfer Function (MFT) may be derived for focus adjustment. As each holder height has a definite value, the corresponding MFT is a discrete number and not very accurate as a whole. Besides, many different heights of holder 11 have to be prepared and setup for testing to obtain the MFT. It is a very time consuming and not precise process. There is still room for improvement.
It is an object of this invention to provide a simple method and device for measuring and testing depth of field of scanner with greater accuracy. According to this invention, a test chart is placed on a slant board which has one side resting on the scanning board and another side supported by a holder of a selected height above the scanning board to form a plurality of scanning zones of different heights. Then the scanning chassis may be moved almost linearly to scan the test chart to obtain the mating MFT and derive a line graph of depth of field distribution, based on which to derive the optimum focus point.
The method and device according to this invention includes a selected test chart, a holder of a selected height, a transparent scanning board, a scanning chassis of fixed optical path which consists of a least one reflection mirror, a lens and a CCD. The test chart is a line pair test chart which has a plurality of diagonal black and white strips. The holder is mounted at one end of the scanning board. The test chart is then held at a slant manner above the scanning board with one end thereof resting on the holder and another end resting on the scanning board. The scanning chassis then may be moved to scan the test chart for obtaining corresponding MTF distribution for adjusting focus use.
During operation, the scanning chassis may be moved horizontally to scan the test chart. The test chart has a plurality of zones formed by multiple transverse and longitudinal lines. Each zone has diagonal black and white strip pairs. Based on the scanning results, an image histogram may be produced. Then MTF may be derived. The MTF calculation equation is:
MTF=(axe2x88x92b)/(a+b)*100%
Where:
a: is maximum histogram value,
b: is minimum histogram value,
A zone that produces greatest value difference between a and b will derive maximum MTF value, and indicates the best focus adjusting point.
Because of the slant test chart of this invention, a continuous linear MTF graph and distribution may be obtained for scanner design and production use to adjust the CCD and lens for the optimum focus.